This invention relates to a method and system for blind channel impulse response combining for noise mitigation in channel estimation in a TD-SCDMA receiver.
Channel estimation is one of the essential function blocks in a TD-SCDMA receiver: it provides instant fading channel parameters to an inner receiver with the aid of the training sequence, called a midamble, carried over each time slot. Since a TD-SCDMA system is an interference limited system, capability of multi-cell joint detection on mobile receivers helps improve the down link performance, e.g., block error rate, ThroughPut etc. This requires a multi-cell channel estimation function block. TD-SCDMA systems may employ beam forming with smart antennas. Thus, signals for different user equipments (UE) in the same cell may experience differently fading channels. Since TD-SCDMA employs Joint Detection (JD), which requires knowledge of all these fading channels, several training sequences may be transmitted simultaneously so that a user equipment can estimate all these fading channels. This is known as default midamble allocation (DMA) mode in TD-SCDMA. With the same total power transmitted from the base station, power allocated to each training sequence in the default midamble allocation scheme is less than that in the scheme associated with only one training sequence for all equipments. Thus, the quality of channel estimation in default midamble allocation mode might be worse. If one user equipment is assigned with several channelization codes, it may occupy several training sequences, although fading channels these training sequences and corresponding channelization codes experience are identical. One way to improve the quality of channel estimation in default midamble allocation cases is to combine the channel estimation results from these midambles belonging to the same user equipments to mitigate the noise. One commonly used combining method is maximum ration combining (MRC). This combining is particularly useful in high speed downlink packet access (HSDPA) scenarios where one user equipment normally takes more than one training sequence.
To carry out this combining action, a user equipment should know the detailed allocation of these midamble sequences. It is not an issue for the midambles allocated to the desired user equipment in a home cell, where such information is available and can be forwarded to the user equipment through a higher layer data. However, for other non-desired midambles such as those from home cell not associated with that user equipment or any midambles from other than the home cell, information is not available at the receiver end. Particularly when multi-cell channel estimation is considered, information of neighboring cell training sequence allocations is not accessible to user equipments in general.